There is well known an atomic layer growing method (hereinafter also abbreviated to an ALD (Atomic Layer Deposition) method) for forming a thin film on a substrate in units of atomic layers. In the ALD, two types of gases that mainly contain elements constituting a film to be deposited are alternately supplied onto the deposition target substrate, and the thin film is formed on the substrate in units of atomic layers. A film having a desired thickness is formed by repeatedly forming the thin film in units of atomic layers. For example, a source gas containing TMA (Tri-Methyl Aluminum) and the oxidizing gas containing O are used when an Al2O3 film is formed on the substrate. The nitriding gas is used instead of the oxidizing gas when a nitride film is formed on the substrate.
A so-called self-stopping action (self-limit function) of growth is utilized in the ALD method. That is, in the self-stopping action of growth, only one or several layers of source gas components are adsorbed on a substrate surface during supply of the source gas, and the excess source gas does not contribute to the growth of the thin film.
The thin film formed by the ALD method has both a high step coverage characteristic and a high film-thickness control characteristic compared with the thin film formed by a general CVD (Chemical Vapor Deposition) method. Therefore, the ALD method is expected to become commonplace in forming a capacitor of a memory element and an insulating film called a “high-k gate”. Because the insulating film can be formed at a low temperature of about 300° C., the ALD method is also expected to be applied to formation of a gate insulating film of a thin film transistor in a display device, such as a liquid crystal display, in which a glass substrate is used.
In the ALD method, the source gas component adsorbed on the substrate is oxidized by utilizing an oxygen radical that is generated by generating the plasma using the oxidizing gas. At this point, the process is performed while a plasma generation time is fixed to hundreds milliseconds to several seconds. For example, Patent Document 1 describes plasma ALD for a tantalum nitride film. In Patent Document 1,a H2 plasma pulse generation time used for reduction is fixedly set to 2000 to 4000 milliseconds (see paragraph [0064] of Patent Document 1).